1. Field of the Invention
The present invention relates to a patch measurement device provided in a printing apparatus, and more particularly to a patch measurement device for measuring color density of patches constituting a control strip which is printed on printing paper.
2. Related Art Statement
There have conventionally been realized printing apparatuses which incorporate a so-called CTP (Computer To Plate) device, i.e., a prepressing device (=a printing plate recording device) that generates an image on a printing plate based on digital image data. A printing apparatus of this type, referred to as a DI (direct imaging) press, is capable of producing printed materials directly from image data, and therefore may be suitable for producing a variety of printed materials, each in relatively small quantities, over short periods of time. While prepress and other processes in such a digital printing apparatus are automated for ease of operation by non-proficient operators, further automation is desired in the control of ink supply, for example, during a printing process.
The control of ink supply in a conventional printing apparatus is generally realized by means of a separate console-type color measurement device, where a produced sample print is measured on a table. In this case, there is a problem in that a human operator needs to take out sample prints from the printing apparatus as necessary to measure the colors appearing on the printed materials.
In order to reduce the amount of work which requires the presence of a human operator as mentioned above, Japanese Patent No. 2824334 discloses a printing apparatus comprising a means for capturing an image of a printed material. In accordance with this printing apparatus, an image of a printed material is captured on an impression cylinder of the printing apparatus, whereby image data is obtained. This image data is compared against reference image data, which is previously read from a printed material that serves as a control reference, and the amount of supplied ink is controlled based on the comparison result. This printing apparatus has an advantage in that there is no need for a human operator as in the case of employing a console-type color measurement device because the printed material is imaged within the printing apparatus.
However, the aforementioned printing apparatus has a problem in that, since an image of the entire printed material must be read for comparison against the reference image, the size of the image data to be handled becomes large, thus requiring a relatively long image data processing time. Since it is necessary to prepare a reference image, this printing apparatus is not suitable for producing relatively few copies of a variety of printed materials, where agility is of the essence.
In order to solve the above problem, a printing apparatus has been proposed which prints a control strip (other than the actual printing image) on a printed material, such that the control strip is measured within the printing apparatus. FIGS. 15A and 15B are diagrams illustrating specific examples of such control strips. Hereinafter, the details of these control strips will be described with reference to FIGS. 15A and 15B.
FIG. 15A is a diagram illustrating a printed material S which may be obtained by using the conventional printing apparatus. As shown in FIG. 15A, the conventional printing apparatus prints an image im on printing paper, and thereafter prints four control strips cs1 to cs4 and three reference marks rm1 to rm3 on the same printing paper. Hereinafter, such four control strips cs1 to cs4 may collectively be referred to as “control strips cs”, and the three reference marks rm1 to rm3 as “reference marks rm”.
The image im is printed on the printing paper, beginning at a position (hereinafter referred to as a “print start position”) which is located a predetermined gripper margin f away from the leading end of the printing paper. More specifically, the image im is progressively printed in the direction of print progress indicated by the arrow (hereinafter referred to as a “first printing direction”), beginning from the print start position. The image im has a dimension m along the first printing direction, which is designated according to the image size. The control strips cs and the reference marks rm are printed beginning at a position which is a predetermined distance n away from the trailing end of the image im.
As shown in FIG. 15A, the control strips cs are typically printed on the printing paper with predetermined intervals therebetween along a direction (hereinafter referred to as a “second printing direction”) perpendicular to the first printing direction, and each control strip cs includes a plurality of rectangular-shaped patches arranged in a predetermined order. Each patch may be a half-tone, linework, or solid image which is printed at a predetermined density in a predetermined color. FIG. 15A illustrates an exemplary patch pc1.
As shown in FIG. 15A, the reference mark rm1 is interposed between two adjoining control strips cs2 and cs3. The reference mark rm2 is interposed between the control strips cs1 and cs2, and the reference mark rm3 is interposed between the control strips cs3 and cs4. As such, the reference marks rm1 to rm3 serve as references based on which to detect the positions of the control strips cs1 to cs4. Typically, as exemplified by the reference mark rm1 shown in FIG. 15B, a reference mark comprises two bars b1 and b2 which run parallel to the first printing direction, and a cross mark c interposed between the bars b1 and b2. Each patch is printed at a position which is predetermined distances away—along the first and second printing directions—from a crosspoint P of the cross mark c. For example, the patch pc1 is printed so that the center thereof is at a distance h (along the first printing direction) and at a distance w (along the second printing direction) from the crosspoint P of the reference mark rm1.
An image of the printed material S is captured by an imaging device provided in the printing apparatus, and is passed as “printed-image data” (i.e., data representing the actually produced printed material) to a patch processing device which is provided in the printing apparatus. Assuming that the patch pc1 is currently to be processed by the patch processing device, the patch processing device first needs to detect the position of the patch pc1 in order to be able to measure the patch pc1. Therefore, the positions of the three reference marks rm1 to rm3 are detected by applying a pattern recognition technique to the reference marks rm, and the position of the patch pc1 is calculated on the basis of the reference mark rm1. The reference marks rm may be detected as follows. Firstly, the patch measurement device previously obtains a pixel array pattern in the vicinity of the crosspoint P1 of the cross mark c interposed between the bars b1 and b2 in each reference mark rm. Next, the patch measurement device checks whether or not this pixel array pattern matches is contained in the imaged printed-image data, while shifting the pixels one by one. The patch measurement device applies such a technique to the printed-image data, individually along the first and second printing direction, and ascertains the coordinates indicating the highest match as a reference mark rm.
Then, the patch processing device first detects the crosspoint P of the reference mark rm1. Furthermore, the patch processing device estimates that a position which is at the patch distance h (along the first printing direction) and at the patch distance w (along the second printing direction) from the detected crosspoint P should be the relative position of the center of the patch pc1, which is currently to be processed. Thereafter, the patch processing device measures the color density information of the patch pc1 at the estimated relative position.
However, the aforementioned technique of detecting reference marks rm through pattern recognition can be hindered by the strong correlation between the pixel array pattern and any pattern present in the printed-image data that resembles the pixel array pattern. In other words, if any pattern resembling the pixel array pattern exists in the neighborhood of a reference mark rm due to flares in the optical system, print smears, and the like, this resembling pattern may erroneously be recognized as a reference mark rm. This results in a failure to detect the actual reference mark rm, and instead a false position will be detected. In particular, since the printed material S is read by the printing apparatus during its transportation, the read position of the printed material S may fluctuate due to recoil and like actions of the printed material S during its transportation. As a result, the detected position of each reference mark rm will be incorrect, so that the wrong position of the patches composing the associated control strip cs will be detected. In such a case, the patch measurement device will try to measure the color density information of a patch in a false position, resulting in the wrong color density information being provided to the printing apparatus. As a result, the ink supply control and the like during the printing process cannot be properly attained. Conventionally, in order to prevent the wrong color density information from being provided to the printing apparatus, a predetermined threshold value is used for determining the validity of the color density information. However, in general, a printing apparatus tends to increase its printing density gradually from the beginning of printing, and the conventional method of employing a threshold value cannot properly address such changes in the printing density.